Radiation meters

ABSTRACT

A radiation meter for measuring the intensity of incident ultraviolet radiation in a predetermined wavelength band against a background of broadband radiation, such as sunlight, includes a sheet of a material incorporating a fluorescent dye. The dye is selected to absorb radiation in the predetermined wavelength band and consequently to emit fluorescent radiation. The fluorescent radiation is internally reflected within the sheet and emerges from an edge of the sheet, where it impinges on a photodiode. The photodiode produces an output which is dependent upon the intensity of the ultraviolet radiation. This output is used to drive a suitable display. The unwanted background radiation passes through the sheet without causing fluorescence of the dye, and is absorbed in an absorbent backing layer. The photodiode is so positioned and screened that the background radiation cannot impinge upon it. A particularly advantageous use of the meter is for measurement of the intensity of ultraviolet band radiation, which is the cause of sunburn and skin cancer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to radiation meters, and in particular toradiation meters for measuring the intensity of incident radiationwithin a predetermined wavelength band against a background of incidentradiation within other wavelength bands, for example broadbandradiation.

2. Description of Related Art

Such meters are widely used in a number of applications by individualconsumers, in industry, and in medicine. One example is in the siliconintegrated circuit industry where high-resolution photolithographicprocesses depend on a source of ultra-violet radiation of knownintensity. In order to distinguish between the incident UV radiation andthe background wideband radiation, the meters presently used in such anapplication employ a filter which selectively transmits UV radiation toa UV-sensitive photo-detector. As both these items are quite expensive,this leads to the cost of such a meter being relatively high.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide such a radiationmeter which does not necessarily require a filter for selectivelytransmitting radiation within a predetermined wavelength band nor adetector which is sensitive to radiation within the predeterminedwavelength band, the meter thus being of lower cost than previouslyknown meters of this type.

According to the present invention there is provided a radiation meterfor measuring the intensity of incident radiation within a selectedfirst wavelength band against a background of incident radiation withina second wavelength band, the radiation meter comprising a body which issubstantially transparent to said incident radiation within the secondwavelength band and which includes fluorescent material having anabsorption spectrum such that the material is effective to select saidfirst wavelength band by absorbing said incident radiation within thefirst wavelength band and consequently emits radiation within a thirdwavelength band different from the first wavelength band, the body beingeffective to direct the emitted radiation on to detector means which isscreened from the incident radiation and which detects the emittedradiation and produces an output representative of the intensity of theradiation detected.

The incident radiation within a second wavelength band may be broadbandradiation. Even so, in a radiation meter in accordance with theinvention the detector means may be a broadband detector.

Preferably the surface of the body on which said incident radiation isincident is larger than the surface of said body to which said emittedradiation is directed.

BRIEF DESCRIPTION OF THE DRAWINGS

Radiation meters in accordance with the invention will now be described,by way of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a pictorial view of the meter;

FIG. 2 is a fragmentary section along a line II--II of FIG. 1 on alarger scale than FIG. 1;

FIG. 3 is a block schematic diagram of electronic circuitry incorporatedin the meter; and

FIG. 4 is a schematic view of an alternative form of part of the meterof FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring firstly to FIGS. 1 and 2, the meter, which is designed tomeasure the intensity of incident UV radiation within the wavelengthband 280-320 nm (UVB) against a background of sunlight, comprises aplastics housing 1, in which is mounted a 3 mm thick rectangular sheet 3of poly(methylmethacrylate), part of the upper surface of which isvisible through a window 4 in the end of the housing 1, which window issubstantially smaller than the sheet 3. Throughout the sheet 3 there isdispersed an amount of approximately 1 to 3% w/w of, for example, one ofthe fluorescent dyes shown in Table I below, the absorption spectra ofwhich closely match the erythemal response curve of human skin and theemission peaks of which lie between 325 and 450 nm.

                  TABLE 1                                                         ______________________________________                                        Dye                Absorption Edge                                            ______________________________________                                        9,10 Dihydrophenanthrene                                                                         302-312 nm                                                 1 Phenylnaphthalene                                                                              302-325 nm                                                 2 Phenylnaphthalene                                                                              300-330 nm                                                 3,7 - Dihydro-s-indacene                                                                         290-325 nm                                                 1,1 - Diphenylbuta-1,3-diene                                                                     300-320 nm                                                 ______________________________________                                    

The sheet 3 is carried within the housing 1 by spacers 5 which separatethe sheet 3 from a matt absorbing layer 7 on a support mounted off theside of the housing 1. A photodiode 9, which is preferably a galliumphosphide device, but may alternatively be a silicon device, is alsoprovided within the housing 1 with its receptive face in contact withpart of the edge of the sheet 3, the rest of the edge being preferablycoated with a layer of white paint 11. The output of the photodiode isconnected via leads 13 to electronic circuitry 15 including a display17, such as a liquid crystal display, which is visible through a furtherwindow in the housing 1. An inwardly-pointing wall 18 around the window4 prevents radiation received through the window from impinging directlyon the photodiode 9.

Referring also to FIG. 3, the electronic circuitry 15 is of standardlow-cost form including an operational amplifier 19 connected by anetwork of resistors and a Zener diode 21 to the display 17, thecircuitry being driven by a small battery 23.

In use of the meter, most of the sunlight 24 incident on the uppersurface of the sheet 3 will pass straight through the sheet to beabsorbed by the matt absorbing layer 7, the air gap between the sheet 3and the layer 7 minimizing the amount of sunlight which will bereflected at the lower surface of the sheet 3. Incident UV radiationwill, however, be absorbed by the fluorescent dye within the sheet 3,which will then emit fluorescent radiation of a longer wavelength. Inthe absence of defects, for example scratches on the surfaces of thesheet 3, a large fraction of this radiation will be totally internallyreflected by the parallel-sided plastic sheet 3, radiation incident onthe reflective layer 11 also being reflected back into the sheet 3.Thus, a concentration of the fluorescent radiation will occur at thereceptive surface of the photodetector 9, giving the meter a high signalto noise ratio. An indication of the output of the detector 9 is givenby the display 17.

It will be appreciated that in the form described above the meter may becheaply fabricated in a relatively compact form, and will thus findready application by the general public for measuring the intensity ofUVB radiation, i.e. the radiation responsible for sunburn and skincancer, so as to give an indication of the amount of time a person maybe safely exposed to the prevailing sunlight. A suitable form ofmaterial for the sheet 3 for this application is available from NuclearEnterprises under the trade name Pilot U. This material already containsan absorbing dye having an absorption band between 240 and 350 nm, andan emission band centered at 390 nm. Such a meter may include a devicefor assisting the user in directing the receptive surface of the sheet 3towards the incident sunlight. Such a device may take the form of asmall peg projecting from the housing 1, effective to cast a shadow onone or more fiducial marks.

A radiation meter in accordance with the invention may be designed forthe measurement of the intensity of incident radiation within anywavelength band. In particular a meter in accordance with the inventionmay find particular application by x-ray workers as a cheap, compactpersonal monitoring system for monitoring the x-radiation level. Such ameter may be made very compact to act as an instantly readablealternative to the film badges presently used by x-ray workers. Anindication that a greater than acceptable intensity of x-radiation isincident on the meter may be given, for example, by an aural signalproduced by a bleeper incorporated in the meter.

It will be appreciated that the choice of fluorescent material willdepend on the wavelength band of the incident radiation which it isrequired to detect, and also on the detector means used to detect thefluorescent radiation, visible light photodetectors such as siliconphotodiodes being particularly applicable as they are very cheap. Afluorescent dye dispersed through a sheet of material which istransparent to the incident radiation which it is not required tomeasure is a particularly convenient medium.

In an alternative form of the device, a primary fluorescent material maybe used to absorb the incident radiation within the chosen wavelengthband, this producing fluorescent radiation which is itself absorbed by asecondary fluorescent material, which in turn fluoresces to produce theradiation detected by the detector means. If the primary fluorescentmaterial selectively absorbs radiation within a narrow selectedwavelength band, whilst the secondary fluorescent material absorbs boththe fluorescent radiation emitted by the primary fluorescent materialand the wideband incident radiation, then the meter may be used to givean indication of the intensity of the total incident radiation and, byinsertion of a single optical filter in front of the photodiode, themeter will indicate the intensity of the radiation within the selectedwavelength band.

In another alternative form, as shown in FIG. 4, the sheet 3 is replacedby two sheets 25 and 27 which are bonded end-to-end in such a mannerthat a good optically-transparent joint is produced therebetween. Thesheet 25 contains a primary fluorescent dye which reponds to theselected (e.g. UVB) radiation wavelength to produce a primaryfluorescence of a given wavelength. This primary fluorescent radiationenters the sheet 27. This sheet contains a secondary fluorescentmaterial which is responsive to the primary fluorescence and which emitsa secondary fluorescence of longer wavelength, which then impinges uponthe photodiode 9. This arrangement enables the use of a photodiode whichneeds only to respond to longer wavelengths and so facilitates theprovision of a suitable photodiode or other photo-sensitive device.

In order to achieve accurate selection of the desired wavelength bandfor detection in any of the embodiments described above, a quantity ofan absorptive dye may be dissolved into the sheet, before or afterincorporation of the fluorescent dye therein, to modify the wavelengthresponse of the sheet. For example, the response of a sheet containing afluorescent dye which responds in a wavelength band which is slightlydifferent from the 280-320 mm band in which the tanning or sunburningresponse of the human skin lies can be matched more accurately to thatresponse by the incorporation of a suitable absorptive dye in the sheet.

We claim:
 1. A radiation meter for measuring the intensity of incidentradiation within a selected first wavelength band in the presence ofincident radiation within a second wavelength band or bands, theradiation meter comprising: a body which is substantially transparent tosaid incident radiation within the second wavelength band or bands andwhich includes fluorescent material having an absorption spectrum suchthat the material is effective to select only said first wavelength bandby absorbing said incident radiation within the first wavelength bandand consequently emitting radiation within a third wavelength banddifferent from the first wavelength band, the body being effective todirect the emitted radiation onto detector means which is screened fromthe incident radiation and which is operative for detecting the emittedradiation and for producing an output representative of the intensity ofthe radiation detected.
 2. A meter according to claim 1, in which saidsecond wavelength band is broadband and the detector means is responsiveto broadband radiation.
 3. A meter according to claim 1, in which thesurface of the body on which the radiation is incident is larger thanthe surface of the body to which the emitted radiation is directed.
 4. Ameter according to claim 1 including display means responsive to saidoutput to give a visible indication of said intensity of radiationdetected.
 5. A meter according to claim 1, in which the body is ofplanar construction such that said radiation within the third wavelengthband is totally internally reflected between two major surfaces of saidbody, the detector means being positioned so as to be responsive toradiation emitted from a peripheral surface of said body.
 6. A meteraccording to claim 5, in which the body comprises a sheet of materialthrough which is dispersed at least one fluorescent dye.
 7. A meteraccording to claim 6, in which said sheet of material is formed frompoly(methylmethacrylate).
 8. A meter according to claim 6, in which thedye is selected from the following dyes: 9,10 Dihydrophenanthrene, 1Phenylnaphthalene, 2 Phenylnaphthalene, 3,7 Dihydro-s-indacene and 1,1Diphenylbuta-1,3-diene.
 9. A meter according to claim 6, including aplurality of said fluorescent materials, each excitable by incidentradiation within a different wavelength band.
 10. A meter according toclaim 9, in which a primary fluorescent material is excitable by saidincident radiation within said first wavelength band, and a secondaryfluorescent material is excitable by the fluorescent radiation emittedby said primary material to produce said radiation within the thirdwavelength band.
 11. A meter according to claim 10, in which the bodycomprises first and second optically-coupled members which contain theprimary and secondary fluorescent materials, respectively; and in whichthe second member is screened from the incident radiation in said firstand second wavelength bands.
 12. A meter according to claim 1, in whichthe body includes an absorptive dye to modify the wavelength response ofthe body to obtain greater accuracy in the selection of said firstwavelength band.
 13. A meter according to claim 1, in which said firstwavelength band is the UVB wavelength band.
 14. A meter according toclaim 1, in which the radiation in said second wavelength band isvisible light.